Chlorine leaving group KIE

Two methods were used to measure the chlorine leaving-group KIE for the 5n2 reduction of benzyl chloride to toluene by sodium borohydride in DMSO at 30 °C. One procedure involved the classical IRMS technique. The second method was a new technique in which the ratio of the chlorine isotopes was obtained by fast atom bombardment mass spectrometry on silver chloride recovered from the reaction. The KIE values found by the two methods were 1.007 and 1.008, respectively,... 

Table 1 The transition structures and chlorine leaving group KIEs for the nucleophiles at 298°C... 

Two reasons seemed possible for the different transition structures found by interpreting the experimental KIEs and theory. One was that the experimental KIEs were measured in solution whereas the theoretical calculations were done for a gas-phase reaction. A second possibility was that the chlorine leaving group KIE was interpreted incorrectly. If one could not conclude the Ca-Cl transition state bond is... 

Table 25 The secondary a-deuterium, the secondary P-deuterium, the a-carbon UC/14C, the incoming group carbon 12C/13C, the nucleophile nitrogen and the chlorine leaving group KIEs found for the Sn2 reaction between ethyl chloride and tetrabutylammonium cyanide in anhydrous DMSO and THF at 30°C...

The same phenomenon, i.e. that the secondary a-deuterium KIE is determined by the changes in only the shorter reacting bond in the SN2 transition state rather than by the nucleophile-leaving group distance, has been found in a completely different reaction system. Matsson, Westaway and co-workers (Matsson et al., 1996) used ku/k14 carbon incoming nucleophile, chlorine leaving group (Hill and Fry, 1962), and ku/ku a-carbon (Fry, 1970) KIEs to model the transition states for a series of SN2 reactions between p-substituted benzyl chlorides and labelled cyanide ion (reaction (17)). 

Table 10 The chlorine leaving group and secondary a-deuterium KIEs and the Hammett p values for the Sn2 reactions between para-substituted benzyl chlorides and thiophenoxide ion in 0.250 M lithium methoxide in methanol at 20°C...

Table 14 The chlorine 35C1/37C1 leaving group, the incoming nucleophile nC/14C, the secondary a-deuterium, and the 12C/14C a-carbon KIEs for the SN2 reactions between p-substituted benzyl chlorides and cyanide ion in 20% aqueous DMSO at 30°C. ...

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